This invention relates to a stepping motor driving circuit to drive a stepping motor.
Conventionally, various types of stepping motor driving circuits have been developed to stably and securely drive a stepping motor. FIG. 1 shows an example of conventional stepping motor driving circuits. In the diagram, coils L1 and L2 are motor coils for the stepping motor and are energized by exciting currents having a mutual phase difference of 180.degree.. These motor coils L1 and L2 are each connected at one end through a choke coil LX and a transistor switching circuit TSC1 to a positive power supply terminal VC, and connected at the other end through a corresponding one of npn transistors TR1 and TR2 to one end of a resistor R1 which is grounded at the other end. A series circuit of a diode D1 and a resistor R2 is connected in parallel to the motor coil L1, and a series circuit of a diode D2 and a resistor R3 is connected in parallel to the motor coil L2.
The transistor switching circuit TSC1 includes a pnp transistor TR3 whose collector and emitter are respectively connected to the choke coil LX and the power supply terminal VC; and npn transistor TR4 whose collector is connected through resistors R4 and R5 to the power supply terminal VC and whose emitter is grounded; and a diode D3 whose cathode is connected to the collector of the transistor TR3 and whose anode is grounded. The connection point between the resistors R4 and R5 is connected to the base of the transistor TR3. The base of the transistor TR4 of the transistor switching circuit TSC1 is connected to a voltage comparing circuit VCP1 for comparing a voltage across the resistor R1 with a reference voltage to be determined in accordance with a control signal CS1 from a drive signal generator DSG. This voltage comparing circuit VCP1 includes a pnp transistor TR5 whose emitter is connected to a positive power supply terminal VE and through a resistor R6 to the base of the transistor TR5 and whose collector is connected through resistors R8 to R10 to the power supply terminal VE and through a resistor R7 to the emitter of the transistor TR5; a comparator CMP1 whose inverting input terminal and non-inverting input terminal are grounded respectively through a capacitor C1 and a resistor R11 and whose output terminal is connected through a resistor R12 to the base of the transistor TR4; and a diode D4 connected between the base of the transistor TR4 and the ground. The output terminal of the comparator CMP1 is also connected to a connection point between the resistors R9 and R10, and the non-inverting input terminal is connected to the connection point between the resistors R8 and R9. A voltage VEE lower than a voltage VCC that will be applied to the power supply terminal VC is applied to the power supply terminal VE.
A drive signal CP1 from the drive signal generator DSG is supplied through an inverter IV1 and a resistor R13 to the base of the transistor TR1, and a drive signal CP1, which is in inversion relation to this drive signal CP1, is supplied through an inverter IV2 and a resistor R14 to the base of the transistor TR2.
This motor driving circuit further includes a motor coil energizing circuit MCEC1 for supplying an exciting current to another pair of motor coils L3 and L4 indicated by the broken lines in FIG. 1 in response to drive signals CP2 a control signal CS2 from the drive signal generator DSG. This motor coil energizing circuit MCEC1 is constituted and operates in the same manner as the above-described motor coil energizing circuit to pass the exciting current through the motor coils L1 and L2.
Next, the operation of the motor driving circuit shown in FIG. 1 will be described.
When the control signal CS1 from the drive signal generator DSG is at a low level, the transistor TR5 is turned on. In this case, a voltage V11 at the noninverting input terminal of the comparator CMP1 is represented by the following equation. ##EQU1## Wherein, VCH is a high level output voltage from the comparator CMP1.
For example, assuming that VCH=15 V, R8=2.4 k.OMEGA., R9=33 k.OMEGA., R11=1k.OMEGA., and VEE=5 V, then V11 is almost equal to 1.75 V.
The transistors TR4 and TR3 are turned on in response to a high level output signal which is generated from the comparator CMP1 by the non-inverting input voltage V11. Thus, the exciting current flows through the motor coil L1 or L2 connected to one of the transistors TR1 and TR2 which is turned on by the drive signal CP1 or CP1, allowing the stepping motor to be driven by one step. For example, now assuming that the drive signal CP1 is at a low level and the transistor TR1 is made conductive, and a current IF1 flows through the transistor TR3, choke coil LX, motor coil L1, transistor TR1, and resistor R1. When this current IF1 is smaller than V11/R1, an output signal of the comparator CMP1 is held at a high level. When the current IF1 is larger than V11/R1, the output signal of the comparator CMP1 reaches a low level VCL, causing the transistors TR4 and TR3 to be turned off. In this case, the input voltage V12 to the non-inverting input terminal of this comparator CMP1 is represented by the following equation. ##EQU2## Assuming VCL=-15 V, then V12=1.13 V. Thus, although a voltage supply from the power supply terminal VC is cut off, the current IF1 continuously flows through the transistor TR1 and resistor R1 due to the current energy accumulated in the choke coil LX and motor coil L1. When this current IF1 becomes smaller than V12/R1, the output signal of the comparator CMP1 again reaches a high level to turn on the transistors TR4 and TR3, so that the exciting current flows again through the motor coil L1. Therefore, the stepping motor is securely held in the present location. In this way, while the control signal CS1 is at the low level, the output signal which reaches a high level periodically is generated from the comparator CMP1, allowing the exciting current to flow periodically through the motor coil L1.
When the control signal CS1 from the drive signal generator DSG1 is at a high level, the transistor TR5 is off. In this case, an input voltage V13 at the noninverting input terminal of the comparator CMP1 is represented by the following equation. ##EQU3## For example, assuming that R7=4.7 k.OMEGA., then, the input voltage V13 is almost equal to 0.99 V.
The transistors TR4 and TR3 are turned on by the high level output signal VCH which is generated from the comparator CMP1 by the non-inverting input voltage V13. Now, assuming that the drive signal CP1 is at a low level and the transistor TR1 is conductive, the current IF1 flows through the transistor TR3, choke coil LX, motor coil L1, transistor TR1, and resistor R1. When this current IF1 gradually increases and becomes larger than V13/R1, the low level output signal VCL is generated from the comparator CMP1. In this case, an input voltage V14 at the non-inverting input terminal of this comparator CMP1 is represented by the following equation. ##EQU4##
Thereafter, when the current IF1 gradually decreases and becomes smaller than V14/R1, the high level output signal VCH is generated from the comparator CMP1. In this way, while the control signal CS1 is at a high level, the output signal which periodically reaches a high level is generated from the comparator CMP1, allowing the current to periodically flow through the motor coil L1, thereby securely holding the stepping motor in the current location.
In the driving circuit shown in FIG. 1, the driving of the stepping motor is controlled by the periodic generation of high level voltage from the comparator CMP1 utilizing the hysteresis characteristics of the voltage comparing circuit VCP1 and the current energy to be accumulated in the choke coil LX and motor coil L1 or L2. However, the current energy accumulated in these motor coils is not constant due to mutual interference between the motor coils L1 and L2 or the like and it is difficult to supply the exciting current at stable periods.